The invention is concerned with ion implanters and with a beam stop used in an ion implanter.
Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting in the wafer ions of required dopant.
Known ion implanters include batch type implanters such as described in U.S. Pat. No. 4,733,091 (assigned to Applied Materials, Inc.), and single wafer-type implanters, such as described in U.S. Pat. No. 5,229,615 (assigned to Eaton Corporation). In typical batch type implanters, wafers being implanted are mechanically scanned in each of two substantially orthogonal directions, repeatedly through a fixed ion beam, to ensure an even implantation dose over the entire wafer surface. In typical single wafer-type implanters, the ion beam itself is scanned transversely in one orthogonal direction at a relatively high scanning rate, and the single wafer being implanted is mechanically translated to and fro across the scanned beam substantially in a second orthogonal direction.
In single wafer-type implanters, the ion beam can be scanned electrostatically or electromagnetically and it is normal practice to collimate the scanned beam so that the beam impinging on the wafer remains parallel to a desired beam direction during scanning.
An object of the present invention is to provide an implanter with provision for setting up the correct alignment and positioning of the scanned beam relative to the wafer to be implanted. A further object of embodiments of the invention is the provision of a beam stop for use in a scanned beam implanter.
Accordingly, in one aspect the invention provides a beam stop for an ion implanter in which the ion beam is scanned in at least one direction transverse to the beam path, the beam stop having a dimension extending in said one direction to receive the beam over its scan in said one direction, and comprising at least one charge collecting member providing a surface exposed to receive ions in said beam, said exposed surface extending in said one direction a distance less than said dimension so that said charge collecting member receives beam ions during only a part of the scan of the beam in said one direction.
With this arrangement, a separate electrical connection can be made to the charge collecting member and the current received by the charge collecting member from the beam as it scans to and fro can be monitored. Because the charge collecting member receives beam ions during only a part of the scan of the beam, the current signals from the charge collecting member has a characteristic shape which is repeated in synchronism with the scanning of the ion beam. By comparing the timing of features of this characteristic shape of the current signal, with the timing of the scanning of the ion beam, the position of the scanned beam relative to the beam stop itself can be monitored.
Preferably, the beam stop includes a beam stop plate extending in said one direction to receive the beam over the scan of the beam in said one direction, said charge collecting member being electrically insulated from the beam stop plate. Then, the beam stop plate may have a surface receiving the beam and the charge collecting member may be mounted behind the surface, the surface having an aperture in front of the charge collecting member to permit beam ions to pass through the surface to impinge on the collecting member. In this way, the exposed surface of the collecting member can be made relatively small in the scanning direction of the beam, with the result that the current signal from the charge collecting member exhibits a characteristic pulse shape as the beam scans over the aperture in the surface of the beam stop plate. The timing of this pulse relative to the beam scanning can then be used for monitoring the location of the scanned beam relative to the beam stop.
The beam stop plate should normally be thick enough to absorb the power of the ion beam impinging on it. The plate will normally be water cooled. Typically the beam stop plate is thicker in the beam direction than the collecting member and then has a cavity behind the aperture in the facing surface of the beam stop plate, with the collecting member being mounted in this cavity.
The charge collecting member may be a rod and said aperture in the facing surface of the beam stop plate may be a slit, both the rod and slit then extending transversely of the beam scanning direction.
In one embodiment, the beam stop comprises a single charge suppressed Faraday cup having an opening extending in said one direction to receive the beam over its scan in said one direction, said charge collecting member being located in said Faraday cup. Then the beam stop plate is also located in the Faraday cup and is usually insulated from the Faraday cup, to allow a current signal to be derived from the plate for monitoring the total beam current absorbed in the beam stop.
In respective preferred embodiments, the beam stop, the beam stop plate or the Faraday cup opening may extend over the entire scan of the beam.
In a preferred arrangement, a plurality of said charge collecting members electrically insulated from each other are provided at different locations along the scanning direction. Individual characteristic current signals can then be derived from each of the charge collecting members for use, for example with a travelling Faraday or other beam detector as will be described later herein, for monitoring that the scanned beam is correctly centered on the beam stop, is aligned with a required beam direction and remains parallel during scanning.
Conveniently three said charge collecting members are distributed symmetrically in the beam scanning direction.
The invention also provides an ion implanter comprising an ion beam generator, a scanner for scanning the ion beam in at least one direction transverse to the beam path, a process chamber receiving the scanned beam and having a process station in the path of the scanned beam at which a substrate can be processed, and a beam end station behind said processing station for terminating the scanned beam and defining a nominal center line for the scanned beam, said beam end station including at least one fixed beam detector providing a characteristic signal as the beam is scanned over the detector. Preferably the implanter further includes a centering monitor responsive to the timing of said characteristic signals from said at least one beam detector to provide a centering signal indicative of the centering of said beam relative to said nominal centre line.
Preferably, the or each fixed beam detector is constituted by a respective charge collecting member in a beam stop of the kind described above.
A plurality of said fixed beam detectors may be distributed symmetrically about said nominal center line along said one direction. Preferably, said scanner is operative to scan the ion beam in accordance with a symmetrical triangular waveform, successive characteristic signals from each of any pair of symmetrically opposed said fixed beam detectors having respective first and second alternating uniform time spacings, and said centering monitor providing said centering signals as a function of any difference between said first and second time spacings for one of said pair of detectors and said first and second time spacings for the other of said pair.
In another arrangement there may be one fixed beam detector located on said nominal center line. Then, again where said scanner provides a symmetrical triangular scan waveform, said centering monitor provides said centering signal as a function of any non-uniformity in the time spacing of the successive characteristic signals from said central fixed beam detector.
In an alternative arrangement, one fixed beam detector may be located at a known distance from said center line and then said centering monitor may be operative to compare the timing of characteristic signals from said beam detector with the waveform of said scan signal.
Preferably, the scanner is operative to deflect the beam in accordance with a periodic scan signal including an adjustable dc component for centering the scanned beam, and the implanter further includes a controller responsive to said centering signal to adjust said dc component to center said beam.
In another embodiment, the implanter further comprises a collimator in combination with said scanner for maintaining the scanned beam path parallel, a travelling beam detector located upstream of said beam end station and movable in said one direction, said travelling beam detector providing a second characteristic signal as the beam is scanned over said travelling detector, and an alignment monitor responsive to a function of the timings of said first-mentioned and said second characteristic signals to provide a beam alignment signal.
Preferably, said scanner and collimator combination is operative to deflect the beam in accordance with a periodic scan signal and is adjustable to adjust the path direction of the parallel scanned beam, and the implanter further includes a controller responsive to said alignment signal for controlling the scanner and collimator combination to adjust said path direction towards a required beam path direction.
Preferably, said travelling beam detector has an actuator responsive to said controller to move said detector to desired positions in said one direction, and said controller is operative to measure the timing of said first characteristic signal with respect to said scan waveform when the travelling beam detector is positioned by said actuator so as not to screen said fixed beam detector, and to measure the timing of said second characteristic signal with respect to said scan waveform when the travelling beam detector is positioned by the actuator at a predetermined location along said one direction, said predetermined location corresponding to the location of said fixed detector along said one direction, said controller being responsive to said measured timings to adjust said path direction so that corresponding said measured timings are substantially the same.
In a further aspect, the present invention provides an ion implanter comprising an ion beam generator, a scanner and collimator for scanning the ion beam in at least one direction transverse to the beam path and for maintaining the scanned beam path parallel, a process chamber receiving the parallel scanned beam and having a process station in the path of the scanned beam at which a substrate can be processed, a beam end station behind said processing station for terminating the scanned beam, said beam end station including at least two fixed beam detectors at respective spaced positions along said one direction, each said beam detector providing a respective first characteristic signal as the beam is scanned over the fixed detector, a travelling beam detector located upstream of said beam end station and movable in said one direction, said travelling beam detector providing a second characteristic signal as the beam is scanned over said travelling detector, and a parallelism monitor responsive to a function of at least the timings, of said first characteristic signals and of said second characteristic signals at at least two spaced positions of the travelling beam detector along said one direction, to provide a collimation signal indicative of the parallelism of said scanned beam.
Preferably then, said travelling beam detector has an actuator responsive to said controller to move said detector to desired position in said one direction, and said controller is operative to measure the timings of said first characteristic signals when the travelling beam detector is positioned by said actuator so as not to screen the respective fixed beam detectors, and to measure the timings of said second characteristic signals when the travelling beam detector is positioned by the actuator at predetermined locations along said one direction corresponding respectively to the locations of said fixed detectors, said parallelism monitor being responsive to said measured timings to provide said collimation signal.
The substrate for processing is typically a semiconductor substrate, but flat panel and polymer substrates may also be used.
There follows by way of example only a description of preferred embodiments of the invention.